Modern metal working and forming processes normally require the use of cutting fluids whose function is to facilitate the machining operations by cooling and lubricating the workpiece. In machining operations where only lubrication is necessary, lube oils are used. When lubrication and cooling are required, lube oil-in-water emulsions are used. The cooling function is accomplished by the ability of the fluid to carry off the heat generated by the frictional contact between the tool and the workpiece. Cooling aids tool life, preserves tool hardness and helps to maintain the dimensions of the machined parts. The cutting fluid also serves to carry away debris from the work area. Both straight lube oil cutting fluids and lube oil-in-water emulsions conventionally contain additives such as defoamants, corrosion inhibitors, extreme pressure active material, metal cleaners, and the like.
In addition to the primary functions of cooling and lubricating, cutting fluids should be non-corrosive, chemically stable, non-toxic and dermatologically safe. Moreover, the cutting fluid composition itself should have some stability against microorganisms. Bacteria and fungi frequently spoil soluble cutting fluids, especially oil in water emulsions, during machining operations. The cutting environment, which contains warm water and an available carbon source, provides a good medium for microorganism growth. Spoilage often manifests itself first as a foul smell. Furthermore, spoilage can cause color change, emulsion break, acidity increase, and sludge formation. Any of these signal a need to change the cutting fluid. Under severe conditions of use, changes due to biological fouling can be required every few days.
To remedy this situation it is conventional in the art to add various biocides to cutting fluid compositions in order to control microbial growth and thus extend fluid life. However, biocides present their own problems. Some cause contact dermatitis, and others cause allergic dermatitis. Still others, through in situ modification, become suspect carcinogens. Government regulations and worker concern about biocide exposure further inhibit their use. Even in situations where the added biocides have no known adverse reaction to humans, biological fouling of the cutting fluid can still occur within a short time and lead to the serious problems of offensive odor, filter plugging, eventual loss of lubricative properties and corrosion of parts and machinery caused by acidic by-products generated by the microorganisms. Furthermore, if the cutting fluid is employed in the form of a stable emulsion with water, biological fouling can lead to the breaking of the emulsion.
Biocides also seriously complicate the problem of the disposal of waste cutting fluid emulsions. For disposal, the emulsion is broken, usually by acidification. The aqueous phase is separated and preferably discharged into a local river. However, biocides in the emulsion can be carried into the aqueous phase when the emulsion is broken, rendering that phase toxic to marine life and requiring costly pretreatment before discharge to the environment.
In view of the foregoing problems associated with the use of biocides in cutting fluids, an ideal cutting fluid would thus contain no biocide while exhibiting bioresistance for prolonged periods of use.
Soluble cutting oils, such as those exemplified by the current industrial standards, are known to degrade biologically, suggesting that oleic acid or other fatty acids, acting as a surfactant in these fluids, contributes most to spoilage by supplying food for microbial growth. As surfactants, sulfonates can replace carboxylates. However, sulfate-reducing bacteria convert sulfonates to hydrogen sulfide, a well known malodorant. In addition, the typical cutting fluid basestock--namely, mineral oil, contains normal paraffins which can also provide food to microorganisms. It is well known that normal paraffinic moieties are biologically degraded at a rate faster than paraffinic moieties containing branching or tertiary carbon atoms.
The observation of the difference in biodegradability of straight chain versus branched aliphatic hydrocarbons has been applied in U.S. patent application Ser. No. 07/441,795, filed Nov. 27, 1989 now and incorporated herein by reference. The discovery of bioresistant surfactants based on isohexadecanoic acid and isoeicosanoic acid which have a branched carbon skeleton is reported therein. Cutting oil emulsion formulations prepared using these acids as surfactant show a high resistance to biodegradation, without incorporating biocides in the formulation. However, the branched carboxylic acids disclosed in the cited application as bioresistant surfactants are relatively expensive and their use compromises the utility of cutting oil formulations prepared therefrom.
It is an objective of the present invention to provide novel bioresistant surfactants derived from relatively inexpensive aliphatic carboxylic acids having a branched carbon chain.
Another object of the present invention is to provide cutting oil formulations using these novel bioresistant surfactants.
A further objective of the present invention is to provide cutting oil formulations incorporating the above novel surfactants together with bioresistant, branch chain lubricants.
Yet another object of the present invention is to provide the foregoing novel cutting oil formulations containing non-biocidal additives to enhance their utility and compatibility with environmental biosystems.